1. Field of the Invention
The present invention relates to a method of forging a crank shaft for molding an integral type crank shaft for an internal combustion engine by forging, under the heating state, a billet which is distributed in capacity into a crank throw portion corresponding to a crank pin and balance weights on both sides thereof and into a journal portion. The invention also relates to a rough forging mold and a finish forging mold used for the forging method.
2. Description of the Prior Art
In the past, it has been general that in forging and molding a crank shaft, a billet consisting of a round bar or a billet which is so shaped as to have a distribution in capacity into a crank throw portion having a large diameter and a journal portion having a small diameter is forged and molded by rough forging and finish forging, after which deflashing is effected.
In a conventional forging and molding as above, in case of a billet consisting of a round bar, the positioning of a billet 40 to a crank pin forming recess 38 and a balance weight forming recess 39 of a lower mold 37 and the positioning of billet 40 to a journal forming recess 41 are uncertain as shown in FIGS. 1 and 2. This is true of a billet which is shaped with a distribution in capacity into a crank throw portion and a journal portion. Also, in the prior art, as shown in FIG. 3, projections 43, 44 are provided on peripheral edges of recesses 38, 39 and 41 of an upper mold 42 and a lower mold 37, and a gap 45 between the projections 43, 44 is made small in order to restrict an outflow of burr 4. However, the gap 45 is formed to be further widened and open outwardly in a horizontal direction so that the effect of restricting the outflow of the burr 4 could hardly been expected.
As described above, the positioning has been uncertain and the outflow of burr great in the prior art. Therefore, the weight of billet has inevitably become heavy and thus the yield has been deteriorated, uneconomically increasing heating energy required and necessitating a large apparatus of high forging pressure.